1. Field of the Invention
The present invention relates to a traction control system for a motor vehicle, and in particular to a traction control system in which a slip value of a driving wheel is controlled by employing an estimated friction coefficient of a road surface.
2. Description of the Related Art
One type of traction control system for a motor vehicle detects a slip value of a driving wheel and controls an output of an engine and braking force so that the slip value of the driving wheel is equal to a predetermined desired slip value. The traction control system is used to prevent a decrease in acceleration of the vehicle caused by the slip which is caused by an excessive driving torque of the driving wheels in an accelerating operation and the like.
The conventional traction control system as shown in Japanese Patent Laid-Open Publication No. 63-263243 controls the output of the engine by using a combination of leaning the air-fuel ratio, retarding the ignition timing and controlling the number of operating cylinders.
In the conventional traction control system, it is difficult to utilize the engine output control for the traction control for the purpose of reducing the work load of the braking system. The reason is that while the driving torque can be reduced rapidly when the braking force is controlled, the driving torque can not be reduced rapidly when the engine output is controlled. Namely, the actual reduction of the output torque of the engine is followed by a certain delay time after the start of the control. Even if the traction control system detects a slip value of the driving wheels and controls the output of the engine, for example, by retarding ignition timing, at the time the actual engine control starts, the slip value of driving wheel already has become close to the predetermined desired slip value by the operation of the braking force. Further, at the time the actual engine control starts, the traction control system detects the current slip value of the driving wheels for a determination of the next engine control. In this condition, the retarding value of the ignition timing is controlled to reduce the value, namely the engine control is controlled toward advancing the timing. Accordingly, hunting of the engine output is caused and the engine output control is not properly operated.